Great Salt Lake, Utah, is a remarkable lake whose history has been carefully studied. It occupies the lowest position of an extensive basin which, in turn, forms but part of the whole great district of Utah which has geologically recently sunk thousands of feet on the west side of the great fault already described as occurring along the western base of the Wasatch Mountains. At present the lake covers about 2,000 square miles, but its area fluctuates considerably. It is scarcely believable that this big lake has an average depth of only fifteen feet and a maximum depth of only fifty feet. It lies 4,200 feet above sea level, and it carries about 18 per cent salts in solution. Most abundant by far is common table salt, of which there are no less than 5,000,000,000 tons in solution. The waters also contain about 900,000,000 tons of other salts. Should the lake completely disappear by evaporation, these salts would be deposited. Allowing for cars 40 feet long and of 40 tons capacity, a train more than 1,000,000 miles long would be required to carry the salts. What has been the source of these salts? Great Salt Lake is not, as supposed by some, a remnant of an ocean once covering the region. Briefly, the explanation is as follows: At one time, when the climate was moister, the basin now only in part occupied by the lake was filled to overflowing with an outlet north into the Snake and Columbia rivers. That great body of water (called "Lake Bonneville") covered nearly 20,000 square miles and its depth was about 1,000 feet deeper than now, the present depth being very small. Because it had an outlet that lake was, of course, fresh. Beaches and shore lines 1,000 feet above the present lake, and at various lower levels, are still wonderfully well preserved. When, due to climatic change, evaporation exceeded intake by streams, the outlet was cut off. But slowly, as the lake shrank, streams (especially the Jordan River) carried a little salt in solution, the percentage of salt increasing until the present stage has been reached. In a real sense, much of the salt was once in the sea, because it has been dissolved out of strata which accumulated under sea water long before the basin of Great Salt Lake came into existence.